1. Field of the Invention
The present invention relates to a brown oxide pretreatment composition for cleaning a copper surface and improving adhesion of a polyimide surface, and a method for improving adhesion of a polyimide surface by applying the composition to a brown oxide process. More particularly, the present invention relates to a composition which comprises an amine, a hydroxide compound, at least one additive selected from a cleaner adjuvant, a antifoaming agent and a precipitation inhibitor; and water wherein the amine and the hydroxide compound act to clean a copper surface and improve adhesion of a polyimide surface, and a method for improving adhesion of a polyimide surface using the composition as a pretreatment for a brown oxide process.
2. Description of the Related Art
Generally, in order to fabricate flexible printed circuit boards, methods for laminating or bonding a single- or double-sided flexible copper clad laminate (FCCL) using a prepreg (PPG) in which an epoxy resin is dipped, have been employed. Strong interfacial adhesion between layers is recognized as one of the most important factors in the final products.
Recently, in order to improve interfacial adhesion between a copper surface and a prepreg, many processes such as a process for forming a conversion coating on a copper surface have been developed. However, flexible PCBs have been fabricated without additional processes for improving interfacial adhesion between a polyimide surface and a prepreg. Thus, there exists a need for methods capable of improving interfacial adhesion between a polyimide surface and a prepreg.
Polyimide is a resin which exhibits excellent heat resistance, mechanical strength, electrical properties, etc., due to its rigid molecular structure. However, polyimide has a problem of delamination in the interface because of its low interfacial adhesion to an epoxy resin. This problem causes defects in the final products and deteriorates the reliability of final products. In particular, the interfacial adhesion is remarkably lowered after a thermal shock test such as a soldering test or hot oil test. Accordingly, improvement in the interfacial adhesion is thought to be a task to be solved for the development of high quality products. In this connection, many studies have been ongoing.
In the electronic device industries, with increasing demand for polyimide films, many processes have been developed. Such processes are largely divided into a dry etching process, a wet etching process and a process using a coupling agent, depending on the characteristics of these processes.
The dry etching process is advantageous in terms of its cleanness. The dry etching process is subdivided into plasma (O2, Ar, NH3), DBD (dielectric barrier discharge), IAR (ion assisted reaction) techniques, etc., which are economically disadvantageous in terms of high cost apparatuses and a low productivity. Advantages of the wet etching process include a relatively low production cost and a high productivity. In the process using a coupling agent, the coupling agent, for example APTMS (aminopropyltrimethoxysilane), improves interfacial adhesion.
Recently, special attention has been paid to the dry etching process owing to its cleanness. Specifically, the dry etching process can improve interfacial adhesion between a polyimide surface and a prepreg by passing a polyimide film through a high energy field under reduced pressure or normal pressure to remove weak boundary layers formed on the polyimide surface and form reactive functional groups. However, as described above, the dry etching process has problems in that high cost apparatuses are required and final products must be stored under stringent conditions for the protection of reactive functional groups formed after surface-treatment.
The wet etching process can improve interfacial adhesion by treating a polyimide surface with an aqueous alkali solution such as KOH (potassium hydroxide) or NaOH (sodium hydroxide) solution, or an amine such as TETA (triethylenetetraamine) or PEHA (pentaethylenehexamine), to make the polyimide surface hydrophilic, and at the same time, introduce reactive functional groups. Although many laboratory studies on the wet etching process have been undertaken, no attempts to apply the wet etching process to the fabrication processes of flexible printed circuit boards have been made because of some problems, for example, complex processes, moisture incorporation and poor reliability.